The present invention generally relates to telecommunication systems and more particularly to a mobile telecommunication system having an expanded operational zone.
In the mobile telecommunication systems for communication with a mobile terminal such as a land mobile radiotelephone, a system called cellular system is used widely. In the cellular system, the service area is divided into a plurality of cells or operational zones formed adjacent with each other, and a base station is provided in each operational zone. The base stations are connected to the public communication network via a common center station and establishes a connection between the public communication network and a mobile terminal via a radio frequency channels. There, the radio frequency used for the communication to the mobile terminal is changed in each zone for avoiding interference, and the center station controls the base stations based upon the information about the mobile terminals operating in the zones. When a connection is to be set up with respect to a particular mobile terminal, the center station controls the base station of the zone in which the mobile terminal is operating to establish a connection therewith. This cellular system is suitable particularly for large cities where there are a large number of subscribers. On the other hand, the cellular system requires a complex control of the base stations such as the registration of the mobile terminals in each zone, switching of the frequency based upon the information about the registered mobile terminals, and the like at the base station. Thereby, the construction of the base station is inevitably complex and expensive.
In the small cities or rural areas where the number of subscribers is small, on the other hand, a more simple, low cost system is desired. In such a low cost system, a single base station is provided to cover a wide service area typically having a diameter of several ten kilometers. Thereby, the facility for controlling the System is simplified significantly.
In such a simplified system, there frequently occurs a demand to expand the service area of the system for example along a major highway. In order to meet such a demand, it is proposed to provide one or more repeater or,relay stations for relaying the communication. Conventionally, such relay stations are connected to the base station via a multiplexed radio channel and operates similarly to the base station except that the relay stations are connected to the public network via the exchange unit of the base station. More specifically, the relay station includes a complex line control unit similar to that of the base station for connection with the public network, in addition to the transmitters and receivers for communication with the mobile terminal. Further, both the base station and relay station are required to have a multiplex microwave radio unit for maintaining the communication therebetween.
FIG. 1 shows the constitution of the aforementioned conventional mobile telecommunication system for rural use.
Referring to FIG. 1, the system generally comprises a base station BS connected to a public communication network 100, a relay station RS connected to the base station via a multiplex microwave radio link MWLINK, and a mobile terminal not illustrated. The base station BS includes an exchange unit 101 connected to the public communication network 100. Thereby, the exchange unit 101 allocates a terminal identification number to each mobile terminal in the service area of the system and establishes an automatic connection between the mobile terminal and the public communication network 100 based upon the terminal identification number.
The base station BS further includes a line control unit 102 connected to the exchange unit for controlling the communication with the base station BS and the mobile terminal that is operating in the operational zone of the base station. More specifically, the line control unit 102 carries out various control functions such as: allocation of frequency to the control channel of the system; calling of the designated mobile terminal via the control channel and allocation of frequency to the talk channel; accepting of a call request from the mobile terminals in the zone via the control channel and allocation of frequency to the talk channel; registration of the terminal location based upon the notification from the mobile terminal via the control channel; switching of the zone while maintaining the connection via the talk channel; connection of the signals in the talk channel, etc.
The line control unit 102 is connected to a number of transmitters 103 and receivers 104 for the talk channels as well as to a transmitter 105 and a receiver 106 for the control channel, and these transmitters and receivers are connected commonly to a non-directional antenna 107 via an antenna duplex and branching unit 108. It should be noted that each transmitter or receiver operates at a frequency pertinent thereto, and these transmitters and receivers are provided in number corresponding to the number of the radio channels of the base station BS.
Further, there is provided a multiplexer/demultiplexer unit 109 in connection with the line control unit 102 for multiplexing and/or demultiplexing the communications to be exchanged between the public network 100 and those terminals outside the zone of the base station BS via a microwave radio link. There, the microwave connection is achieved by a microwave transmitter/receiver unit 110 of the base station BS and a corresponding microwave transmitter/receiver unit 111 of the relay station RS. It should be noted that the communication achieved by 10 the microwave connection includes, in addition to the usual talk channels and the corresponding control channel, a zone Switching control signal for switching the zone between the base station and the relay station. This zone switching control signal is used when a mobile terminal moves from the zone of the base station BS to the zone of the relay station RS, or vice versa.
In the relay station RS, a multiplexer/demultiplexer unit 112 corresponding to the unit 109 of the base station BS is provided, and a line control unit 113 is connected to the unit 112 similarly to the base station. There, the line control unit 113 has a construction and function substantially identical with the line control unit 102 of the base station and controls a number of transmitters and receivers 114 and 115 corresponding to the talk channel as well as a transmitter 116 and a receiver 117 for the control channel. These transmitters and receivers are connected commonly to a non-directional antenna 118 via an antenna sharing unit 119 corresponding to the unit 108 of the base station BS, and achieves a communication with the mobile terminals operating in the operational zone of the relay station RS.
As will be understood easily, the system as shown in FIG. 1 requires complex facilities for the relay terminal RS such as the line control unit 113 that are substantially identical with the corresponding facilities of the base station, in addition to the multiplexer/demultiplexer and multichannel radio units. Thus, the expansion of the service area of the system by adding the relay station shown in FIG. 1 inevitably increases the cost of the system.
Meanwhile, it is known to expand the service area of a radiotelephone beyond the base station as described in the REPORTS OF THE CCIR 1990, ANNEX TO VOLUME IX-PART 1, pp. 345, CCIR, Geneva 1990.
Referring to FIG. 2 showing the construction disclosed in the above mentioned reference somewhat schematically, there is provided a base station BS in connection with a public communication network. The base station has an operational zone #0, and establishes a connection with a stationary, non-mobile terminal S.sub.0 operating in the zone #0. More specifically, the base station BS has a non-directional main antenna and broadcasts in the zone #0 with a frequency f.sub.A, and the broadcast is received by the stationary terminal S.sub.0. The stationary terminal S.sub.0 has a high-gain directional antenna and transmits a radio signal back to the base station BS at a second frequency f.sub.B.
The broadcast of the base station BS at the frequency f.sub.A is received also by a relay station RS.sub.1 that has a directional antenna directing toward the main antenna of the base station BS. Further, the relay station RS.sub.1 transmits back a radio signal to the base station BS via the same directional antenna at a frequency set equal to the frequency f.sub.B and establishes thereby a connection with the base station BS. In addition, the relay station RS.sub.1 has a non-directional main antenna and broadcasts at a frequency set equal to the frequency f.sub.B such that the relay station RS.sub.1 covers an operational zone #1. In the zone #1, there is provided a stationary, non-mobile terminal S.sub.1 that has a directional antenna directing toward the main antenna of the relay station RS.sub.1, and the terminal S.sub.1 transmits a radio signal to the relay station RS.sub.1 at a frequency set equal to the frequency f.sub.A.
Similarly, there is provided another relay station RS.sub.2 wherein the relay station receives the broadcast of the station RS.sub.1 at the frequency f.sub.B and rebroadcasts the same at a frequency set equal to f.sub.A to cover an operational zone #3. Thereby, a transmission from a stationary, non-mobile terminal in the zone #2 is transmitted back to the relay station RS.sub.2 at the frequency f.sub.B and sent back to the relay station RS.sub.1 at the frequency f.sub.A.
In the radiotelephone system of this prior art, the expansion of the service area of the system is achieved easily by constructing additional relay stations such as a relay station RS.sub.3. As the relay station of this Conventional system is required to have only a pair of receivers for receiving the radio signals at the frequencies of f.sub.A and f.sub.B and a pair of transmitters for transmitting the radio signals at the frequencies of f.sub.A and f.sub.B for each channel, the relay station can be constructed very simply. Further, the conventional system of FIG. 2 is advantageous from the view point of the saving of the radio frequency, as the system uses only two frequencies, f.sub.A and f.sub.B, for one channel.
When the system of FIG. 2 is applied to the mobile telecommunication systems wherein the mobile terminal moves around in the zone as well as across the boundary of the zones, there arises a problem in that the transmission from the mobile terminal such as the terminal S.sub.1 in the zone #1 at the frequency f.sub.A may be intercepted by the directional antenna used at the station RS.sub.1 for receiving the broadcast from the base station BS also at the frequency f.sub.A. When this occurs, the transmission from the terminal S.sub.1 is broadcasted again from the main antenna of the relay station RS.sub.1, and the system no longer operates properly.